Xylene isomerization



United States Patent O 3,363,017 XYLENE ISOMERIZATION Harold J. Hepp and Marvin M. Johnson, Bartlesville,

Okla, assignors to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed Dec. 28, 1964, Ser. No. 421,709 7 Claims. (Cl. 260668) ABSTRACT OF THE DISCLOSURE Xylenes are isomerized to produce maximum yields of ortho and para isomers using conventional Xylene isomerization catalysts, particularly those comprising silicaalumina, and at pressures of at least 1000 atmospheres.

This invention relates to an improved process for isomerizing xylenes so as to produce maximum yields of ortho and para isomers.

Para-xylene has for some time been an article of commerce. Hitherto, para-xylene was obtained from petroleum and coal tar xylene fractions, normally consisting essentially of ortho-, meta-, and para-xylenes and ethyl benzene, the content of the para-xylene varying between 10 and percent. More recently, the development of synthetic fabrics such as Dacron has increased the demand for para-xylene to such an extent that the supply of naturally occurring xylenes is insufiicient. Thus, considerable interest has been focused upon the preferential conversion of other hydrocarbons to para-xylene.

It has been known that the isomeric forms of xylene, namely the ortho-xylene, meta-xylene and para-xylene, can be converted one into the other by isomerization. However, that isomerization has been difficult to accomplish and when the isomerization has been carried out under conditions such as would give a relatively selective and clean cut isomerization, the reaction has taken place at such a slow rate as to make it impractical for commercial operation. When the isomerization was carried out under relatively drastic conditions leading to appreciable degradation of the product, the para-xylene yields were low. The xylenes have been isomerized by purely thermal means and by treatment under certain conditions with Friedel-Crafts type catalyst, e.g., aluminum chloride plus hydrogen chloride, but the required conditions were severe and the yields were poor.

Further development of the isomerization of xylenes resulted in the use of clay type cracking catalysts. Preferred catalysts of that type have been the acid treated clays or synthetic materials composed largely of silica in combination with alumina, magnesia, zirconia, or boric oxide. Such catalysts are described, for example, in US. Patents 2,741,646 and 2,818,451. However, even when equilibrium isomerization is achieved the percentage of the para isomer is relatively low at the operating pressures heretofore usedi.e., below about 150 atmospheres.

This invention is concerned with isomerizing a mixed xylenes stream to produce maximum yields of the ortho and para isomers.

Accordingly, it is an object of the invention to provide an improved process for isomerizing xylenes principally to paraand ortho-xylenes. Another object is to provide an improved process for isomerizing a mixed xylenes stream comprising principally meta-xylene, to paraand ortho-xylenes. A further object is to provide an improved xylene isomerization process which maximizes yields of ortho and para isomers and reduces disproportionation. Other objects of the invention will become apparent to one skilled in the art upon consideration of the accompanying disclosure.

The invention comprises isomerizing a mixed xylenes Patented Jan. 9, 1968 stream in intimate contact with a xylene isomerization catalyst at a pressure greater than 1000 atmospheres and, preferably, greater than 3000 atmospheres. It has been found that the yield of the para isomer when operating at these high pressures is about twice that obtained at conventional operating pressures. The yield of the ortho isomer is also almost doubled, and disproportionation reactions leading to the formation of toluene and the trimethyl benzenes are greatly reduced. Disproportionation is further reduced or eliminated by inclusion of about 110 volume percent of toluene in the feed to the isomerization zone.

Other operation conditions for the process of the invention include a temperature of 200 to 800 F., preferably 350 to 650 F., and liquid hourly space velocity of 0.3 to 10, preferably 0.5 to 6.

In the process of the invention, a stream of mixed xylenes, produced by naphtha hydroforming for example, and optionally containing ethylbenzene, is fed to a highpressure, liquid-phase isomerization zone, the effiuent is fed to a fractional crystallization zone for removal of para-xylene, and the balance of the material-together with make-up fresh mixed xylenes feedis returned to the isomerization zone. Optionally, if the ortho isomer is also desired as product, the isomerization zone is followed by a fractionation zone to remove the ortho-xylene as a kettle product, the fractional crystallization zone is used to separate the para-xylene, and the meta-Xylene together with make-up fresh mixed xylenes feedis returned to the isomerization zone. Ortho-xylene can be used in the production of phthalic anhydride by oxidation and is thus at times a very desirable product.

Pressure, atm 1 670 5,400

Xylene isomers, mol percent:

Meta 58 53 12 15 18 30 27 29 58 It is evident that little change in isomer content is effected by increasing operating pressure from 1 to 670 atm., but that a considerable change is effected by a further increase in operating pressure to 5400 atm.

While silica-alumina is the preferred catalyst, other isomerization catalysts such as those disclosed in US. Patents 2,741,646 and 2,818,451 are within the scope of the invention. The essential feature of the invention is the pressure at which the isomerization is conducted, i.e., greater than 1000 atmospheres and, preferably, greater than 3000 atmospheres.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

We claim:

1. A process comprising catalytically isomerizing a feed of mixed xylenes predominantly to the para isomer which comprises intimately contacting said xylenes at a pressure of at least 1000 atmospheres and a temperature in the range of 200 to 800 F. with a xylene isomerization catalyst comprising silica in combination with at least one of the group: alumina, magnesia, zirconia, and boric oxide.

2. The process of claim 1 wherein the catalyst consists principally of silica-alumiua.

3. The process of claim 1 wherein the pressure is at least 3000 atmospheres.

4. The process of claim 1 wherein toluene in the range of 1 to 10 volume percent of the feed is incorporated therein to further reduce disproportionation.

5. A process for iso-merizing a feed stream of mixed xylenes comprising orthoand meta-Xylenes predominantly to para-xylene which comprises intimately contacting said feed in an isomerization zone with an isomerization catalyst effective in isomerizing xylenes comprising silicaalumina, at a pressure of at least 1000 atmospheres, a temperature in the range of 200 to 800 F., and a liquid hourly space velocity in the range of about 0.3 to 10, so as to isomerize said xylene feed predominantly to paraxylene.

6. The process of claim 5 wherein said feed is predominantly meta-xylene and the isomerized stream consists principally of orthoand para-xylenes, and including the steps of:

(a) fractionally distilling said isomerized stream to recover an ortho-xylene-rich bottoms stream and an overhead stream rich in para-xylene and containing a substantial concentration of meta-xylene;

(b) subjecting the overhead stream of step (a) to fractional crystallization to recover para-Xylene as the solid phase and metaexylene as the liquid phase;

(c) and passing the recovered meta-Xylene of step (b) and additional feed stream to said isomerization zone.

7. The process of claim 5 wherein said feed is predominantly meta-xylene and the isomerized stream consists principally of orthoand para-xylenes, and including the steps of:

(a) passing the isomerized stream into a fractional crystallization zone and subjecting same to fractional crystallization to recover para-xylene as a solid phase and metaand ortho-xylenes as a liquid phase; and

(b) recycling the liquid phase of step (a) to said isomerization zone.

References Cited UNITED STATES PATENTS 20 2,564,388 8/1951 Bennett et al. 260668 2,656,397 10/1953 Holzman et al. 260-668 2,775,628 12/1956 Nicholson Ct 211. 260-668 DELBERT E. GANTZ, Primary Examiner.

C. R. DAVIS, Assistant Examiner. 

